Recently, an assembled battery for an industrial use and an in-vehicle use, which uses a high-output battery, has been spreading quickly. In the assembled battery that is used for these purposes, there is a demand for continuing high performance (battery characteristics) over a long period of 10 to 15 years. Accordingly, there is a demand for individual batteries used in the assembled battery to have, for example, the following performance.
First, even when the assembled battery is used under harsh conditions over a long period of 10 to 15 years, there is a demand that deterioration in performance of an individual battery be small. Furthermore, the performance of the assembled battery in which a number of batteries are connected is greatly affected by battery performance of one battery having the lowest performance in the assembled battery. Thus, a variance in the battery performance of the individual batteries, which occurs due to deterioration of the battery performance during a long period of use, is demanded to be small. In addition, there is a demand for the individual batteries to secure satisfactory corrosion resistance for a long period of time.
As a technology in the related art, for example, the following Patent Documents 1 to 5 are exemplary examples.
Patent Document 1 discloses a thionyl chloride lithium battery that is used as a battery for an electronic apparatus. In this thionyl chloride lithium battery, a thin groove portion is provided at a part of a battery lid or a battery case, and a trapezoidal groove (score) on an inner side of a can is shown as a thin groove portion. In Patent Document 1, in a case where inner pressure increases during abnormality in a battery, the thin groove portion is allowed to be ruptured at low pressure, whereby the thin groove portion is used as a safety valve.
Patent Document 2 discloses a square-type sealed battery having a thin groove portion along a width direction of a sealing lid. The thin groove portion has a V-shaped cross-section, and is formed by press working.
Patent Document 3 discloses a cylindrical storage battery in which a rupture valve is formed in a diaphragm that is inversed due to an increase in inner pressure and blocks a current. In the cylindrical storage battery, a groove, which has a V-shaped cross-section and which is constituted by a combination of a circular curve positioned at a surface opposite to a surface of the diaphragm to which the inner pressure is applied, and a straight line of which one end intersects the circular curve and the other end is located on an outer periphery side of the diaphragm in relation to the circular curve, is formed as the rupture valve. In addition, in the cylindrical storage battery, a groove having a U-shaped cross-section is formed in a surface of the diaphragm to which the inner pressure is applied and which corresponds to the position at which the groove having the V-shaped cross-section is formed. In the straight groove, the depth on the other end side is shallower than the depth on the one end side. Furthermore, the depth of the groove having the U-shaped cross-section is shallower than the depth of the groove having the V-shaped cross-section.
Patent Document 4 and Patent Document 5 disclose an easy-open can lid that is opened by rupturing an opening formed in a can lid of a can body so the contents can be taken out, and a production method thereof. The easy-open can lid is different from the battery can in the technical field, and is used as a can lid for a beverage can or a food can. A resin-coated steel sheet is applied as a material of the can lid.
In the assembled battery (storage battery) for industrial use and in-vehicle use, which uses a high-output battery, there is a demand for a battery can capable of satisfying the characteristics necessary for the battery. The present inventors have found that it is necessary to consider the following performance for the battery can.
For example, performance stability of a safety valve during a long period of use is necessary for the battery can. Factors deteriorating the performance stability are as follows: a pressure variation in a battery can due to repetition of charging and discharging (that is, fatigue of the safety valve), and an effect by the contents such as an electrolytic solution (that is, corrosion of the safety valve). In addition, with regard to an individual battery can when being used as an assembled battery, it is necessary for the battery can to have a small variance in a valve opening pressure of the safety valve (that is, uniformity in performance of the safety valve).
However, the thionyl chloride lithium battery that is a primary battery does not perform charging and discharging, and thus in the battery can disclosed in Patent Document 1, it is not necessary to consider fatigue resistance of the safety valve. In addition, the primary battery for an electronic apparatus hardly ever constitutes an assembled battery, and thus a demand for uniformity in performance of the safety valve with respect to a plurality of batteries is not so high. Accordingly, in Patent Document 1, an expensive stainless steel sheet or nickel steel sheet, which has high corrosion resistance, is used, and a trapezoidal score groove is formed in one surface (inner surface of a battery) of the sheet in such a manner that the valve opening pressure may be adjusted by changing a width of the score groove.
In addition, in the square-type battery for portable apparatuses, which is disclosed in Patent Document 2, a life cycle of the apparatuses that are used is short, and is frequently shorter than 10 years, and thus a demand for the fatigue resistance with respect to the safety valve or uniformity in performance of the safety valve with respect to a plurality of batteries is not so high. Accordingly, in Patent Document 2, a V-shaped thin groove portion is used as the safety valve in order to progress a crack from the thin groove portion.
In Patent Document 3, the storage battery has a battery can in which a rupture valve is ruptured after inversion of the diaphragm, and to realize this function, a rupture valve is configured by a groove having a V-shaped cross-section, and a U-shaped groove which is opposite to the groove having a V-shaped cross-section. Accordingly, in the storage battery disclosed in Patent Document 3, a demand for the fatigue resistance with respect to the rupture valve or uniformity in performance of the safety valve with respect to a plurality of batteries is not so high.
In the easy-open can (hereinafter, referred to as an EOE can) disclosed in Patent Document 4 and Patent Document 5, a structure of the can lid is different from that of a portion of the battery can which is provided with a safety valve, and particularly, a can opening mechanism itself is completely different from a mechanism of the battery can provided with the safety valve.
That is, in Patent Document 4 and Patent Document 5, a large force is caused to operate on a local point of a score contour by a tab that functions as a lever, the one local point of the score contour is opened, and then the majority of the score contour is opened by pushing force of the tab. In this manner, in Patent Document 4, a can lid having a mechanical opening function is used. Accordingly, with regard to a mechanical function of opening a can and can opening characteristics that are necessary, the can lid is basically different from the battery can provided with the safety valve.
A difference between a can opening technology in the field of beverage can, and a valve opening technology of the battery can will be described below.
The safety valve of the battery can is reliably opened at a point of time at which inner pressure generated during charging reaches a predetermined designed inner pressure, and the battery can itself is prevented from being ruptured. When the designed inner pressure of the safety valve is too high, the battery inner pressure increases too much, and thus the battery can is apt to be ruptured. On the other hand, when the designed inner pressure of the safety valve is too low, the safety valve is opened during charging, and thus an assembled battery itself is broken. Accordingly, it is necessary to limit the upper limit of an amount of charge, and thus the performance of the assembled battery is greatly limited.
On the other hand, the beverage can uses a method (a technical field in which a can opening mechanism is completely different from a mechanism of the safety valve of the battery can) in which force is focused on one point of the score portion by a component called a tab functioning as a lever to allow large force to operate so as to open the can. Accordingly, performance necessary for this method (that is, performance of the beverage can) includes durability against an impact, easiness of can opening, and mass productivity. With regard to the durability against an impact, for example, even when a can into which contents are put is dropped several times at a height of approximately 1 m, it is necessary for the durability to have a level at which a score portion is not opened by deformation of the can due to the dropping and the contents do not flow out. This level is sufficiently lower than a level necessary for the battery can, and thus a remaining score thickness (Min value) may be made to be small to the limit of a level necessary for the durability against an impact. In addition, with regard to the easiness of the can opening, it is necessary to set the upper limit of can opening power to a remaining score thickness level capable of being opened by the tip of a finger of human. The can opening power has a variation depending on kinds of cans, makers, and the like, and has a large margin within a setting range of the can opening power (a technical field having a margin for the setting of the remaining score thickness). Accordingly, in the beverage can, the can opening power may vary in an individual beverage can, and it is not necessary to increase the processing accuracy to a certain degree of the battery can to increase mass productivity.
In addition, with regard to the safety valve of the battery can, it is necessary for the score portion of the safety valve not to be fatigue-broken under conditions in which large inner pressure is repetitively applied to the battery can at all times over 10 to 15 years, and it is necessary for the safety valve to be opened at a point of time at which inner pressure set as valve opening pressure is loaded. On the other hand, in the beverage can, the repetitive load itself is not present. Accordingly, the technical field of the beverage can is a technical field in which it is not necessary to consider fatigue strength to which the present inventors give attention.
Accordingly, the technology of the beverage can does not have the same technical idea as the battery can which endures a repetitive stress load over a long period of time, and which is reliably opened at a point of time at which a predetermined loaded inner pressure is applied.
Also, as disclosed in Patent Document 5, in a case where the bottom of the score groove has a curvature, large can-opening-power is necessary, and thus can opening properties decrease. Accordingly, in this case, it is necessary to extremely decrease the remaining score thickness.
In addition, in the safety valve of the battery can, it is necessary for the valve opening pressure itself not to be too much lower than a set value, and not to be too much higher than the set value, that is, it is necessary for a variance in an absolute value of the valve opening pressure of a number of battery cans to be small. Furthermore, since an assembled battery is designed in such a manner that the valve opening pressure is common to all of assembled batteries that are produced, it is necessary for individual battery cans used in all of the assembled batteries to have a small variance in the valve opening pressure of the safety valve of the battery can. As mentioned above, with regard to the battery can, accuracy (reproducibility) of the safety valve of the individual battery can is a very important technical field. However, the beverage can belongs to a technical field having no such technical idea.
As mentioned above, the valve opening technology of the battery can is completely different from the can opening technology of the beverage can.